A solid polymer electrolyte (hereinafter referred to as a "SPE") is a polymer substance which exhibits a high ion conductivity in the solid state, and the application thereof to various sensors and fuel cells is anticipated.
Furthermore, the SPE can be applied to a next generation battery, a photoelectric cell or an electrochromic element.
In order to impart high ion conductivity to a SPE, the SPE must have a low glass transition temperature. However, when the glass transition temperature is low, the film strength is lowered which makes the SPE difficult to handle in an industrial environment.
Furthermore, in order to improve the ion conductivity, a method of adding an organic solvent is often used. However, this causes a reduction in strength which makes the SPE even more difficult to handle.
Accordingly, at present, a method of coating a SPE prepolymer directly on an electrode, followed by cross-linking and solidification, is commonly used.
However, the above-described method encounters difficulty in controlling the polymerization degree of the SPE or the thickness of the SPE film, and a homogenous and uniform SPE film can hardly be obtained.
For these reasons, it has been difficult to obtain a SPE film comprising a SPE material having a low glass transition temperature, a high ion conductivity, a homogeneous ion conductivity and a uniform thickness. It has also been difficult to composite a SPE layer by laminating the SPE layer on an electrode such that the layer homogeneously adheres to the electrode to provide good electrical contact between the SPE layer and the electrode.
Since a SPE may have a water-absorbing property depending on the kind of SPE, the atmosphere should be controlled in order to control the water content during the step of compositing the SPE and an electrode.